The goal of this K99/R00 project is to test the hypothesis that genotoxic stress in the nucleus triggers signaling events that result in accumulation of dysfunctional mitochondria, which in turn drives cellular senescence and aging. The hypothesis is supported by preliminary data demonstrating that depletion of the DNA repair endonuclease ERCC1-XPF in cells and mice causes accumulation of oxidative DNA damage, premature cellular senescence and aging, but also surprisingly mitochondrial dysfunction and increased reactive oxygen species. Moreover, ERCC1-deficient C. elegans also show evidence of mitochondrial dysfunction. ERCC1- XPF is required only for the repair of the nuclear genome, suggesting that nuclear stress can drive mitochondrial abnormalities. Similar observations have been made in murine models and human cells of ataxia telangiectasia and Hutchinson-Gilford Progeria syndrome. Herein we propose to define the molecular mechanism(s) by which nuclear genomic instability triggers mitochondrial dysfunction using an innovative combination of powerful genetic tools. The significance of these studies is the possibility of identifying novel signaling mechanisms that could be targeted therapeutically to prevent cell senescence, aging and age-related diseases arising as a consequence of stochastic damage to cells. The approach will be a genome-wide RNAi screen in ercc-1 C. elegans to identify genes that suppress Complex 1 dysfunction in mutant worms. This unbiased approach will yield pathways that impact mitochondrial function in response to endogenous genotoxic stress and undoubtedly new hypotheses about mechanisms of aging. This will fuel my transition to becoming an independent investigator, a second important goal of this project. A targeted preliminary screen established the feasibility of the approach and revealed several genes critical for the DNA damage response and mitophagy, including ATM, p53, DRP1 and PINK1, that regulate mitochondrial function in ercc-1 worms. These novel links between the nucleus and mitochondria identified in nematodes will be pursued in mice and murine cells. The innovative approach of exploiting the strengths of two very powerful model systems will allow identification of novel molecular targets and support rapid translation of this new knowledge to human aging. The majority of individuals over the age of 65 years suffer from at least two chronic degenerative diseases. These chronic diseases of the elderly consume an increasingly large fraction of our health care costs and rob individuals of their independence and quality of life. Developing therapies to target the primary risk factor for all of these diseases, aging itself, is promising yet challenging solution. The first step is to define the molecular mechanisms that drive aging. This project aims to identify the signaling mechanisms that drive aging in response to endogenous DNA damage, which accumulates in all of us over time. This will reveal novel therapeutic targets that can be exploited to extend healthspan.